1. Field of the Invention
The present invention pertains generally to the field of radiation detecting devices and, more particularly, to the field of real-time radiation imaging devices.
2. Discussion of the Background
There are several instances in modern radiotherapy where real-time imaging of X rays is a highly useful and critically important technique.
In external beam megavoltage photon radiation therapy, it is highly desirable that the maximum dose be delivered to the target volume and the minimum dose be delivered to the surrounding tissue. Prior to treatment, which typically consists of irradiating the patient on a daily basis for several weeks, the patient undergoes a number of preparatory steps in order to identify the region to be irradiated and to determine a "treatment plan" specifying exactly how this irradiation is to be performed. Often, one of these steps is to place the patient on a "treatment simulator", which simulates the motions and geometry of the therapy machine, and which makes diagnostic quality fluoroscopic and radiographic x-ray images. The fluoroscopic imaging allows a real-time means of simultaneously observing patient anatomy and manipulating the position of the patient so as to achieve a desired patient orientation with respect to the simulated treatment beam. Unfortunately, current fluoroscopic devices use large cumbersome image-intensifier tubes which restrict the possible motions of the simulator, thereby limiting the treatment positions that can be simulated.
A permanent record of imaging information from the simulator is achieved by means of radiographic imaging with film as well as storage of the fluoroscopic images. These images are used to provide information which contributes to deciding what the target region should be and how the actual treatment is performed, i.e., what geometric and dosimetric combination of megavoltage beams to use to satisfactorily irradiate the target region but spare the surrounding normal tissues. Once a treatment plan has been determined, often with the assistance of a computer which allows, among other things, manipulation of the simulation information as well as CT or other imaging information, the patient is typically taken back to the simulator for a verification-simulation in order to verify the geometric correctness of the plan.
When the patient is brought into the treatment room, it is highly desirable, prior to treatment, to verify that the orientation of the patient with respect to the treatment beam closely coincides with the setup achieved in the simulator room. Once verified, the prescription dose can be delivered to the target volume and surrounding tissues. The achievement of this goal is complicated by the fact that the patient anatomy moves due to both voluntary and involuntary patient motions. Such complications encourage the possibility of delivering too little dose to the target region and/or overdosing the surrounding tissues. In addition, for treatment machines which use a computer-controlled scanning treatment beam, there is the additional uncertainty of whether the beam is correctly directed on a burst by burst basis.
The above problems can be overcome by real-time imaging. Several prototype real-time imagers are being developed around the world, but most have no practical applications to clinical use. A real-time clinical image detector has been developed by H. Meertens at the Netherlands Cancer Institute in Amsterdam which is disclosed in European Patent Application 0196138. The Meertens' device operates on the principle of a scanning liquid ionization chamber. However, the Meertens' device is able to detect only a small fraction of the imaging signal.
Radiation detecting devices are taught in Hynecek, U.S. Pat. No. 4,679,212; Luderer et al., U.S. Pat. No. 4,250,385; DiBianca, U.S. Pat. No. 4,707,608; Haque, U.S. Pat. No. 4,288,264; Kruger, U.S. Pat. No. Re. 32,164; Barnes, U.S. Pat. No. 4,626,688; and DiBianca et al., U.S. Pat. No. 4,525,628; however, these detectors do not make possible real-time imaging for megavoltage photons.
Imaging equipment has been developed based on the use of photostimulable phosphors wherein an image receptor plate coated with such phosphors is exposed to a radiation beam and then "read out" by means of laser stimulated luminescence with direct conversion of the light to digital form. However, this technology appears to offer no possibility of real-time imaging. Efforts to develop imagers based on camera-fluoroscopy combinations have produced images of greatly varying quality at rates ranging from two images a second to one image every eight seconds. However, such a camera's expensive and delicate imaging electronics would be irreversibly damaged after approximately 10-130 kilorads of dose. Thus, mirrors are used to reflect the light image produced by a metal-phosphor screen combination to a camera sitting outside of the direct radiation field. This makes necessary the presence of a bulky light box located in the vicinity of the treatment table where such obstructions are highly undesirable. Furthermore, with the camera's imaging surface 2 to 3 feet from the fluorescent screen, the solid angle subtended by the camera is small (less than 1%) and hence the image quality is limited by the light collection stage rather than by the available high-energy quanta.
Recently, an imager consisting of tightly packed, tapered, optical fibers has been reported. The fibers make up a 40.times.40 cm.sup.2 surface, 12 cm thick, which sits in the beam behind a metal-fluorescence screen and "pipes" the light to a video camera. The optical fibers are bunched together in bundles of 1.5.times.1.5 cm.sup.2 at the input end and the imager has a thickness of 12 cm. The optical fibers have to be bent to such an extent that light is lost due to the fact that the critical angle is exceeded. The system currently has a light collection efficiency no greater than that of the mirror-camera system and, like those systems, is rather bulky.
In the optical imaging systems discussed above, considerably less than 1% of the visible light photons emitted by the scintillating layer are converted into signal. As a direct consequence, the quantum sink is the light collection stage rather than the stage where X rays are converted to high-energy electrons which enter into the phosphor. Thus, the quality and speed of imaging in the above systems are adversely affected.
In selecting the materials for a real-time imager for megavoltage photon radiation therapy, care must be taken that the materials can withstand high levels of radiation exposure over long durations of time. Another consideration is that the radiation detecting elements be arranged over a relatively large surface area. For instance, a detection surface of at least 25.times.25 cm.sup.2 is necessary for head and neck portals. For pelvic, abdominal and thoracic portals, a surface area of 50.times.50 cm.sup.2 is desirable. Though solid state imagers are highly desirable, the manufacture of crystalline semiconductor detectors over such an area is certainly prohibitively expensive.
The development of a--Si:H (hydrogenated amorphous silicon) has resulted in the realization of a highly radiation resistant material which can be utilized over large surface areas at very economical cost. See V. Perez-Mendez, et al., "Signal, Recombination Effects and Noise in Amorphous Silicon Detectors", Nuclear Instrument and Methods in Physics Research A260 (1987) 195-200, Elsevier Science Publishers B. V.; and I. D. French et al., "The Effect of .gamma.-Irradiation on Amorphous Silicon Field Effect Transistors", Applied Physics A31, 19-22, 1983, Springer-Verlag.
It is now realized that amorphous silicon thin film transistors have applications to large-area electronics, see H. C. Tuan, "Amorphous Silicon Thin Film Transistor and its Application to Large-Area Electronics," Mat. Res. Soc. Symp. Proc. Vol. 33 (1984) Elsevier Science Publishing Company, Inc.
Amorphous silicon ionizing particle detectors made of hydrogenated amorphous silicon are known which can detect the presence, position and amount of high energy ionizing particles, see Street et al, U.S. Pat. No. 4,785,186; however, the patent does not teach how a--Si:H photodiodes can be utilized in coordination with other elements to obtain a real-time imaging device.
Rougeot, U.S. Pat. No. 4,799,094, teaches a photosensitive device having an array of p-doped floating grids which connect with a substrate of lightly n-doped hydrogenated amorphous silicon. Since Rougeot uses transistors as light detectors, the quantity of electron-hole pairs generated would appear quite insufficient to realize real-time imaging.